Methane production is critically important to a number of human-related problems including global warming, waste treatment, and the development of alternative fuels from biological materials. The bulk of methane on earth is produced by methanogenic archaea through the mechanism known as methanogenesis. An essential step in methanogenesis is regeneration of a CoB-CoM heterodi- sulfide formed upon methane production. The enzyme complex responsible for CoB-CoM reduction is known as heterodisulfide reductcase (Hdr). The genome of Methanosarcina acetivorans contains ORFs coding for a soluble HdrABC complex and a membrane-bound HdrDE complex. Whether HdrABC and HdrDE have specialized, or completely redundant functions will be tested by creatng reporter gene fusions, targeted gene deletions, and chimeric Hdr complexes. In vitro analysis of chimeric enzyme complexes will be used to identify amino acid residues important for specific and selective protein:protein interactions. Through these experiments, the mechanism of methanogen- esis and the archaeal electron transport system will be elucidated, thereby gaining insight into the processes of energy generation and conservation that are essential to all forms of life.